In recent years, portable communication devices have become commonplace. These devices use a Radio Frequency (RF) transceiver to send and receive RF signals that typically include audio information. The RF transceiver includes a modulator and demodulator. The modulator modulates a carrier signal with a baseband signal for transmission, and demodulates received signals to obtain a transmitted baseband signal. Baseband signals can be decoded into an audio signal that is further processed and played by an audio circuit via a speaker so that the audio signal can be heard by a user of the device.
Many communication devices include a high-audio speaker to play the audio signal at higher volume levels, such as in a speakerphone application, or in a two-way handheld radio unit. A power amplifier is generally coupled to the speaker to amplify the signal sufficiently such that the user can adequately hear the output audio without having to hold the device to the user's ear, as is typical in a cellular phone. The high audio speaker is a transducer which converts electrical audio signals to mechanical movements of a transducer element to produce acoustic signals in the air.
The acoustic transducer creates significantly higher pressure levels compared to low level audio transducers, such as, for example, telephone earpiece speakers. Accordingly, a large amount of force is required to move the air at the diaphragm where the amount of force is a function of the size of the diaphragm and the size of the magnet. The forceful movement of the diaphragm at high audio levels can also push air into and out of the handset creating pressure which results in audio frequency vibrations in the handset device. Also, when the handset is not optimally enclosed or sealed, the internal acoustic pressure can couple acoustic vibrations into other portions of the device. The problem is noticeably worse when the speaker is in close proximity to the circuit board and electrical/electronic components of the device. All devices and components internal to the handset can be subject to these vibrations. These vibrations can induce bending of component boards such as those that house the RF modulation circuitry.
The electro-mechanical-acoustical stress and strain bending of the boards can change the electrical properties of the integrated circuits which can in turn alter the behavior properties of the device. For an RF component such as a Voltage Control Oscillator (VCO), the mechanical bending can vary the voltage, and, the VCO frequency deviates in relation to the vibration. The deviation effectively superimposes properties of the acoustic signal onto the demodulated signal. In effect, the vibration can modulate the behavior of the demodulator where the result can be regeneration of the output audio on top of the demodulated signal. This behavior is a feedback loop which can oscillate and be unstable when the signals become highly correlated, or in phase. In effect, the regenerative audio feedback acts as a parasitic modulation that gets demodulated and amplified over and over causing oscillatory feedback, commonly called ‘microphonics’. The internal pressure is inversely proportional to the internal air volume. And, as devices become smaller the microphonics problem can continue to increase. Accordingly, a smaller device can go unstable at high volumes which causes a howling effect in the audio signal as a result of receiver audio regeneration.
Current approaches to avoid the bending of the circuit boards include material padding to absorb the sound, mechanical ribs or clips to limit the allowable degree of mechanical bending, and non-piezoelectric capacitors. The current approaches attempt to minimize the acoustic pressure build-up and/or isolate the acoustic coupling. They rely on mechanical solutions that cannot fully resolve the howling problem caused by the regenerative audio feedback. In addition, system engineers set a specification margin for certain parameters in shipping radios to account for tolerances in parts and variances in temperature. However, this lowers the overall volume gain of the handset. A final recourse, when the mechanical solutions are insufficiently capable of mitigating the howling behavior, is to lower the level of high audio speaker output by setting a maximum volume level corresponding to a gain specification level below which howling occurs. Accordingly, the device is shipped with a reduced loudness gain to meet the gain specification margin. However, this reduces the overall loudness level which users expect from a high audio speaker handset. In a public safety environment, or other high ambient noise condition, such restriction may not be acceptable.
Accordingly, there is a need for a method and apparatus for receiving and playing a signal in a radio receiver that avoids the problems associated with the prior art.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.